The alt.usage.english FAQ

"none is" vs "none are"

(Usage Disputes)

With mass nouns, you have to use the singular. ("None of the
wheat is...") With count nouns, you can use either the singular or
the plural. ("None of the books is..." or "None of the books
are...") Usually, the plural sounds more natural, unless you're
trying to emphasize the idea of "not one", or if the words that
follow work better in the singular.
The fullest (prescriptive) treatment is in Eric Partridge's book
_Usage and Abusage_ (Penguin, 1970, 0-14-051024-9). In the original
edition Partridge had prescribed the singular in certain cases, but
a rather long-winded letter from a correspondent persuaded him to
retract.